I picked up YoYo Games' Game Maker Studio last November (wow has it been a year already?!) when it was briefly given away for free. While Multimedia Fusion 2 put me off these sorts of game creation packages (MMF2 has a LOT going for it but scripting what should be quite straightforward stuff can be very convoluted... more so than just simply programming it in fact!) there's such an impressive catalogue of respected games made with GMS that I had to give it a try.
And it's really good, sporting a superb way of treating code like just another resource in a way that lets you click together the framework of a game before filling in the details with scripts. I remade Vaders with it earlier this year as a quick test project and the experience was very enjoyable, which I think is the main thing to say about GameMaker Studio – it makes the creation process fun :)
Anyway, Ludum Dare is fast approaching and a quick squiz at the YoYo Games site reveals that the base version of GMS is currently (permanently?) free so it seemed timely to mention it. The tutorial projects should get you going pretty quickly (hint hint!)
EDIT: Ooh! More exciting Ludum Dare related freeness... it seems the AGK App Game Kit is featured in the current Ludum Dare deals! I'll definitely be giving it a look.